According to rapid development of electrical, electronic, telecommunicational, and computer industry, the need for a secondary cell having a high performance and stability has been increased. Especially, electronic goods are becoming smaller and more portable so that a secondary cell which is a main part thereof also required to be thinner and smaller. Now, although a nickel-cadmium secondary cell and a nickel-hydrogen secondary cell are commonly used, a lithium secondary cell is spotlighted as a new-type secondary cell having higher energy density than those secondary cells.
According to applied electrolytes in a lithium secondary cell, there are a lithium ion cell in which electrolyte is liquid, and a lithium polymer cell in which electrolyte is solid or gel. In case of lithium ion cell there is the merit of high capacity, but it needs special device such as a protective circuit because a liquid electrolyte containing lithium salt should be used. While in case of lithium polymer cell, it is much more stable than lithium ion cell because it uses a polymer as an electrolyte.
In order to be technically available, a lithium polymer secondary cell should be electrochemically stable, that is, it should be stable in the broad range of voltage to resist for an overrecharged condition. And it should show a high ionic conductivity over 1 mS/cm at room temperature. Also, materials for a gel polymer electrolyte which are chemically and electrically compatible with electrolyte materials should be developed. Especially, it should have a high physical property, processability and a good adhesion property between cathode and anode.
It is focused on the development of a gel polymer electrolyte which is made to a polymer electrolyte by mixing with liquid electrolyte used in the system of lithium ion cell to keep the lithium ion conductivity over 0.5 mS/cm at room temperature as a composition of electrolyte. Thus, recently a lithium polymer secondary cell has been developed by Hitachi Maxell Co. and Battery Engineering Co. The electrolyte used in the cell is also gel polymer electrolyte containing liquid electrolyte (Nikkei Electronics, May 19, 1997). These polymer electrolytes are manufactured by addition of a liquid electrolyte to a polymer matrix, which is the most closed system to commercial use of a lithium polymer secondary cell. The typical examples of polymer used as a gel polymer electrolyte are polyacrylonitrile (U.S. Pat. No. 5,219,679), polyvinylidene fluoride (U.S. Pat. No. 5,296,318), etc. Also, a gel polymer electrolyte made by mixing organic electrolyte with copolymer containing chlorovinyl, vinylacetate, acrylonitrile, styrene or methylacrylate was applied to a lithium polymer cell (Japanese Patent Laid-open No. 06-131423). The polymer material copolymerized a high polar monomer such as vinyl chloride, methyl metacrylate, vinyl alcohol, acrylic acid, etc. with a low polar monomer such as styrene, butadiene, etc. was used as a gel polymer electrolyte (Japanese Patent Laid-open No. 06-153127). NBR (acrylonitrile butadiene rubber), ABS (terpolymer of acrylonitrile, butadiene and styrene), and AAS (acrylonitrile acyl styrene copolymer) using acrylonitrile as a basic material was used as an example of the gel polymer electrolyte (Japanese Patent Laid-open No. 06-153127). By analyzing the prior art on the basis of the gel polymer electrolyte material, the most preferable polymer material in use of the electrolyte is (1) a polymer which has a high mechanical strength in order to make an easy preparation of an electronic cell, (2) a gel polymer electrolyte which inhibits the exudation phenomenon of liquid electrolyte out of polymer, (3) a gel polymer having the ionic conductivity over 0.5 mS/cm, (4) a gel polymer electrolyte having a high adhesion between cathode and anode, and (5) a gel polymer electrolyte having a high film formability. However, there is no polymer which has all of the above characteristics among the prior invented polymers. For example, in case of a poly(chlorovinylidene/hexachloropropylene) of U.S. Pat. No. 5,296,318, although the polymer is spotlighted due to a good mechanical strength, film formability, and ionic conductivity, it has difficulty in applying to the method to prepare the cell by sandwiching the gel polymer electrolyte between cathode and anode as a material of the electrolyte, which is generally the well-known method for preparation of a lithium polymer cell. Therefore, in order to improve the polymer material of U.S. Pat. No. 5,296,318, the polymer material was applied to the method of plastic lithium ionic cell preparation (PLI method) in which polymer matrix, cathode film and anode film are laminated to minimize the resistance of interface(The Electrochemical Society Proceedings, Vol. 94-28, pp 330-335).
Accordingly, the present inventors have developed a gel polymer electrolyte whose polymer is selected from the group consisting of poly(vinyl acetate), poly(ethylene/vinyl acetate), poly(ethylene/vinyl acetate/carbon monoxide), and poly(ethylene/vinyl acetate/maleic acid anhydride). The polymer elcectrolytes show a high adhesion, a good ionic conductivity, a good film formability, etc.